The present invention relates to a beam separating prism and more particularly to an endoscope attachment that enables rays of light coming from the eyepiece section of an endoscope to be used in viewing with the naked eye or imaging with a video camera or the like.
Rays of light entering a beam separating prism in one direction pass through it and emerge therefrom as two split beams that are perpendicular to each other. A prism having this beam splitting capability and which is of the simplest design is a cubic half-mirror prism 1 of the type shown in FIG. 5. If this half-mirror prism 1 is disposed in such a way that the half-mirror plane 1a is inclined at 45 degrees with respect to an incident light beam, rays of light that meet the entrance end face 1b in a perpendicular direction are partly transmitted through the prism and emerge from the first exit end face 1c in a perpendicular direction, with the remainder of the incident rays being reflected by the half-mirror plane 1a and emerging from the second exit end face 1d, again in a perpendicular direction.
A problem with the prism shown in FIG. 5 is that the image formed by the rays of light reflected from the half-mirror plane 1a is inverted with respect to the one formed by the rays of light being transmitted through the half-mirror plane 1a.
In an endoscope attachment that uses rays of light coming from the eyepiece of an endoscope in viewing with the naked eye or imaging with a video camera, an upright image needs to be finally formed in both the viewing and imaging optics. Therefore, if two beams issuing from a beam splitting prism could be directly used to form an upright image without letting them pass through any optical element that produces an inverted image, certain advantages would be offered not only in terms of the number of optical components but also in terms of the size of the overall equipment.
It has been proposed that a penta prism capable of even-numbered reflections for beam separation be used and the optics for an endoscope attachment be constructed as shown in FIG. 6.
Rays of light coming from the eyepiece section 2 of an endoscope pass through spacers 3 and 4 and enter a penta prism 5 through an entrance end face 5a; part of them are transmitted straight through the first reflecting plane 5b and emerge from the first exit end face 5c, with the remaining rays being reflected by the first reflecting plane 5b and the second reflecting plane 5d and emerging from the second exit end face 5e.
Although not shown, the beam emerging from the first exit end face 5c is guided into the optics for viewing with the naked eye having an optical axis l.sub.1 which coincides with the optical axis of the endoscope, whereas the beam emerging from the second exit end face 5e is guided into the optics for imaging with a video camera having an optical axis l.sub.2 which is perpendicular to the optical axis l.sub.1.
The arrangement described above has the advantage that an upright image can be formed in both the viewing and imaging optics without employing any optical component that causes image inversion. In this arrangement, the optical axes l.sub.1 and l.sub.2 of the viewing and imaging optics are made perpendicular to each other in order to facilitate the working for rendering the system waterproof.
In consideration of the imaging lens barrel and the thickness of the endoscope, the distance, OP, from the final end face of the eyepiece section 2 in the endoscope attachment described above to the optical axis, l.sub.2, of the imaging optics is set to the smallest value (in this case, 14.25 mm).
On the other hand, in terms of the field of vision that can be attained with the attachment optics for viewing with the naked eye, the distance between the eyepiece section 2 of the endoscope and the first exit end face 5c of the penta prism 5 is desirably as short as possible since as the length of an optical path along the axis l.sub.1 increases, the smaller portion of the rays of light in the marginal area will fall on the viewer's eye.
However, in the endoscope attachment that uses the penta prism 5 shown in FIG. 6, point Q at which the optical axis l.sub.1 intersects the first reflecting plane 5b is positioned closer to the first exit end face 5c than point P where the optical axis l.sub.1 intersects the optical axis l.sub.2, so if the distance, OP, between the eyepiece section 2 and the optical axis l.sub.2 of the imaging optics is preliminarily determined, the distance OR between the eyepiece section 2 and the first exit end face 5c of the penta prism 5 cannot be made reasonably short (in the case being considered, OR is 19.5 mm).